1. Field of the Invention
The invention relates to a filter disc for magnetic separators.
2. The Prior Art
It is well-known that liquids contaminated with suspended particles or dissolved high-molecular substances, for example resinous substances, can be cleaned by adding a ferromagnetic particulate material, for example magnetite, iron, cobalt or nickel, to the liquid and separating the contamination together with the ferromagnetic material in a magnetic field. It is also known to clean in a similar way liquids which, from the start, contain contaminations of ferromagnetic particulate material, for example coolant leaving machine tools, such as lathes and drills. Contaminated gases can also be cleaned in principle by the addition of a ferromagnetic particulate material and treatment in a magnetic field for separation of the contaminations. It is also known to use chemical flocking agents simultaneously in the cleaning process, for example lime, alum, iron chloride, polyelectrolytes and water glass.
In the above described cleaning operations a magnetic separator is used. One such separator (shown for example in Lundquist application Ser. No. 440,872, filed Feb. 8, 1974), now U.S. Pat. No. 3,959,145 is built up of a number of substantially parallel filter discs which are attached, radially directed and spaced from each other, along a rotatable shaft. The filter discs contain permanent magnets producing magnetic field gradients in the spaces located between the filter discs. The medium which is to be filtered and which, when entering the separator, contains a ferromagnetic particulate material, is caused to pass the spaces, which causes the contaminations to adhere to the walls of the filter discs. In a known embodiment of such a filter disc, the permanent magnets are arranged in close contact with each other between two wall discs which act as limit walls for the filter disc in the lateral direction. The permanent magnets can be fixed to the wall disc by a glue, for example an epoxy resin glue.
The use of filter discs of the kind described with glued magnets has proved to involve certain problems. One problem is that the bond between the magnets and the wall discs is not in all respects satisfactory. This is particularly true when using ceramic magnets when the bond between the magnet and the glue may be insufficient to withstand the shear forces which occur when the separator is in operation. The shear forces may be caused by bending forces on the filter discs because of turbulence in the water or another medium which is to be subjected to cleaning, or by the effect of a scraper by which the material adhering to the discs is removed. Another problem with these filter discs is connected with their manufacture. When pressing wall discs with intermediate magnets against each other in connection with the curing of the glue, it has proved to be difficult to retain the magnets in their intended positions. During pressing part of the glue flows from the space between the wall discs outwards towards the periphery, and it has then been found to be difficult to prevent the magnets from following the glue to a certain extent.